Our purpose in this investigation is to ascertain how the pituitary vascular system, whose structure we have previously described, actually functions. We propose to study the direction of blood flow within the pituitary employing microcine photography following the intracarotid injection of lissamine green. The occurrence of circular blood flow within the pituitary implies that adenohypophyseal and neurohypophyseal hormones are delivered to the median eminence where the neurohypophyseal capillary bed is specialized--apparently to deliver hormones to the pars distalis, to the hypothalamus, and to the brain via the ventricular system. Our previous anatomic studies have emphasized the role of the neurohypophyseal capillary bed in the exchange of humoral messages between the adenohypophysis and the brain. We have characterized the volume of blood flow in the ovine neurohypophyseal capillary bed and the effect of changes in arterial pCO2 and MABP upon neurohypophyseal blood flow. We have investigated the relationship between neurohypophyseal blood flow an neurosecretion. We propose to extend these inquiries to relate regional neurohypophyseal blood flow and regional substrate utilization to common neuroendocrine events (such as ovulation). The proposed investigation will increase our understanding of neural control of pituitary function and of short loop feedback, and the means by which the pituitary can secrete to the brain to modulate brain function.